Orientation and loading condition dependence of fracture toughness in cortical bone

The fracture toughness at crack initiation were determined for bovine corti cal bone under tension (mode I), shear (mode II), and tear (mode III). A to tal of 140 compact tension specimens, compact shear specimens and triple pa ntleg (TP) specimens were used to measure fracture toughness under tension, shear, and tear, respectively. Multiple-sample compliance method was utili zed to measure the critical strain energy release rate (G(c)) at the a/W= 0 .55 (crack length, a, to specimen width, W, ratio). The critical stress int ensity factor (K-c) was also calculates from the critical loading (P-c) of the specimens at the a/W= 0.55. The effect of the anisotropy of bone on its resistance to crack initiation under shear and tear loading was investigat ed as well. Fracture toughness of bone with precrack orientations parallel (designed as longitudinal fracture) and Vertical (designed as transverse fr acture) to the longitudinal axis of bone were compared. In longitudinal fra cture, the critical strain energy release rate (G(c)) of cortical bone unde r tension, shear, and tear was 644 +/- 102, 2430 +/- 836, and 1723 +/- 486 N/m, respectively. In transverse fracture, the critical strain energy relea se rate (G(c)) of cortical bone under tension, shear, and tear was 1374 +/- 183, 4710 +/- 1284, and 4016 +/- 948 N/m, respectively. An unpaired t-test analysis demonstrated that the crack initiation fracture toughness of bone under shear and tear loading were significantly greater than that under te nsile loading in both longitudinal and transverse fracture (P < 0.0001 for all). Our results also suggest that cortical bone has been ''designed'' to prevent crack initiation in transverse fracture under tension, shear, and t ear. (C) 2000 Elsevier Science S.A. All rights reserved.